This invention relates to current sensors for duplicating the function of a current transformer, and more particularly, to a modular current sensor and power source.
Current transformers are used to perform various functions in electrical circuits. Current transformers may be disposed on a primary electrical circuit to provide variable electrical power to a secondary electrical circuit. Current transformers may also be used as a sensor to sense electrical current in a primary electrical circuit and provide a signal indicative of the magnitude of the current to a secondary electrical circuit. In some applications, a single current transformer is used to perform both of these functions. However, there are fundamental limitations to the performance of conventional iron core current transformers. Iron core current transformers designed for wide dynamic range require large, heavy and costly iron cores and copper conductors. Further, the accuracy and linearity of iron core current transformers is inherently limited by saturation and magnetic losses. As a result, conventional iron core current transformers tend to be expensive, with somewhat limited accuracy.
One application where a single current transformer may be used to provide both operating power and a current signal to a secondary circuit is in an electrical circuit breaker having an electronic trip unit. Electronic trip units are employed in industrial-rated circuit breakers for a wide variety of protection and other accessory functions. One such electronic trip unit is described in U.S. Pat. No. 4,672,501 entitled Circuit Breaker and Protective Relay Unit.
An advantage of using a single current transformer to perform both power and sensing functions is the simplicity of a two-wire connection between the current transformers and the sensing circuitry (e.g. the trip unit). The sensing circuitry receives the sensing signal and power from two wires. One example of an efficient current transformer used for both sensing and power functions is described in U.S. Pat. No. 4,591,942 entitled Current Sensing Transformer Assembly.
Various patents describe the use of two devices, one for power and one for sensing, to overcome the aforementioned design limitations inherent in using a single transformer. For example, the use of the combination of an air core transformer for current sensing and an iron core transformer for providing operating power is found in U.S. Pat. No. 5,583,732 entitled Modular Current Transformer for Electronic Circuit Interrupters. U.S. Pat. No. 5,615,075 entitled AC/DC Current Sensor for a Circuit Breaker, describes the use of a Hall effect device for current sensing in combination with an auxiliary power supply for providing operating power to the trip unit processor. However, previously, when sensing devices such as an air core current transformer, Hall effect device and the like are employed for the current sensing function, four wires have been required, two for providing power from the power transformer to the power supply circuitry and two for providing signals from the sensing device to the sensing circuitry. The added wires can increase the cost to manufacture new devices. Moreover, the need for additional wires precludes using such current sensors with existing applications having a two conductor input.
In an exemplary embodiment of the invention, a modular current sensor and power source unit are combined in a configuration duplicating the function of a current transformer. The invention consists of a sensing circuit, a power source, a power supply, and a current source combined and connected to an electrical distribution circuit and electrical device. In this case a current transformer is used as the power source. A power supply circuit connects with the current transformer for regulating the operational current input. A current sensor circuit is arranged for connection with the electrical distribution circuit for providing a signal indicative of current flow through the electrical distribution circuit. A current source circuit connects with the current sensor circuit for receiving the current signal and with the power supply circuit for receiving the operational current from the power supply circuit. The current source circuit connects with an electrical device for providing the operational current and the current signal to the electrical device.